The present invention relates to a display device for three-dimensional representation of a 3D scene, which is visible for an observer eye from at least one visibility region. The display device includes a controllable device for tracking the at least one visibility region in an observer plane of the display device, where the main component of said tracking device is based on a controllable optical waveguide. The display device further comprises a controllable spatial light modulator (SLM) with a pixel matrix for modulating the light, and a system controller unit for controlling the electrode arrangement. The tracking device comprises at least one light source which emits light, the controllable waveguide for guiding the light, an electrode arrangement, and an lens array with lenses.
The display device can for example be used as an autostereoscopic or holographic display for the three-dimensional representation of holographic or stereoscopic video sequences. Used as a holographic display device, it serves for generating a coherent plane wave field using the light which is coupled out of the waveguide. The wave field is directed at a controllable spatial light modulator means (SLM) on which e.g. a hologram of a scene is encoded with complex values. In the holographic display device, the SLM often serves as the holographic screen at the same time. The display device can be used for 3D representations for multiple observers. The display device can also be used in autostereoscopic displays.
A coherent plane two-dimensional wave field which exhibits sufficient temporal and spatial coherence is needed for generating a holographic reconstruction of a spatial scene in a holographic display device. This means that a planar wave field with a sufficiently small plane wave spectrum is to be realised with the help of light sources. Lasers, which are known to emit coherent light, are generally used as light sources. Alternatively, a multitude of LEDs arranged in a matrix, which normally emit incoherent light, can be used as light sources. If the light which is emitted by the LEDs is filtered spatially and/or temporally, it will be given the sufficient coherence which is required for holographic representations. However, the larger the diagonal of a controllable spatial light modulator (SLM), the greater the demands made on the coherence and representation quality in the holographic display device.
An illumination device in a holographic display device requires a certain number of point light sources or, for a one-dimensionally encoded hologram, a certain number of line light sources to achieve an efficient illumination in order to generate a visibility region.
For certain tracking methods, which take advantage of controllable electrowetting prisms (EW prisms) arranged in a prism array, the light sources can have a fix position. The light follows an observer in that the angles of the EW prisms are variable and adjusted accordingly by the controller. For other tracking methods, e.g. such methods that take advantage of light source tracking, the position of those light sources must be variable.
A known solution for a light source tracking is to use a backlight in conjunction with a shutter display, as described in document EP 1776614 (A1) filed by the applicant. In a shutter display, individual pixels or pixel groups can be controlled such to be switched to a transmissive mode, while others are switched to an absorbing mode. The transmissive pixels serve as light sources. Individual pixels are preferably used for holographic full-parallax encoding, and line-shaped groups of pixels are preferably used for holographic single-parallax encoding on the SLM of the holographic display device or for stereoscopic 3D display devices. Alternating positions of light source images can be realised by controlling the shutter pixels accordingly.
However, a great disadvantage of such a display device is the very low efficiency in respect to brightness, because the backlight must continuously supply a constant luminous intensity across the entire surface of the shutter panel, while those pixels of the shutter panel which are not switched to the transmissive mode absorb a large portion of the light.
Document U.S. Pat. No. 6,816,140 B2 describes a display device with an LC display panel which takes advantage of LC waveguides, in which light propagates as a result of total internal reflections and which leaves the waveguide towards an observer when a certain voltage is applied. The user can watch displayed information from any position in front of the display. It is the purpose of this arrangement to make the LC display in a flat and lightweight design and as inexpensive as possible. However, a generation and tracking of a visibility region according to the movement of the user does not take place.
It is further commonly known to use a compact surface-emitting light waveguide for illuminating an SLM in a display device. The waveguide is for example a compact slab made of transparent plastic material, where the light is transmitted into the narrow side face of the slab. The thickness of the transparent slab can for example vary as the slab exhibits a wedge-shaped angle between its two large surfaces, and the surface which faces the display panel can have a structure of micro-prisms. This design serves to achieve a preferred polarisation of the light to be emitted. In order to increase the portion of the useful light, it is known to apply a depolarising diffuser foil on the back surface of the plastic slab. This is also referred to a polarisation recycling. The multiply reflected light is emitted from the entire surface of such a waveguide. The angular range of the light that leaves the waveguide is for example around 30°, and is by a factor of 1800 larger than the angular resolution of the human eye. This type of light waveguide is not suited for generating a plane wave field which is meant to illuminate an SLM and e.g. to generate a holographic reconstruction. The light beams must only contain portions of plane waves which mutually diverge by an angle of ≦ 1/20° if they are collimated to form a plane wave field.
It is further known to use as fix light sources striped waveguides which are designed in the form of parallel stripes to form a compact backlight and which exhibit at defined fix positions output coupling points for the injected light. This backlight for illuminating the SLM has a higher efficiency than the above-mentioned surface-emitting light waveguide, but due to its concept it is restricted to fix light source positions. In this configuration, the backlight can preferably be used in conjunction with an array of EW prisms for tracking. Only light sources with a low spectral range, i.e. mainly lasers, can be used for EW prism tracking though. In contrast, light source tracking would permit the use of more broad-band light sources, such as LEDs. However, the positions of the light sources needed to be variable here.
As is generally known, the striped waveguides are based on the principle of total reflection of the injected light. The light is guided within a medium (core) with a higher refractive index, which is surrounded by a cladding made of a different material with a lower refractive index.
Document [1] U.S. Pat. No. 3,980,395 discloses a switch for optical waveguides based on LC material to be used in communications technology. The light propagates in a waveguide whose core is made of a material with a refractive index N1. The core is surrounded by a substrate with a lower refractive index N0 on the one side, and by a birefringent liquid crystal material with the refractive index N2 on the other side. The birefringent material is connected with spatially structured electrodes. When a voltage is applied to the electrodes, the effective refractive index of the birefringent material is changed in the region between the controlled electrodes, such that light can be injected into or coupled out of the waveguide.
However, using birefringent materials such as liquid crystals has the disadvantage that the effective refractive index also depends on the angle under which the light hits the interface of the two materials. The refractive index difference can thus vary among the individual modes of a multi-mode waveguide.
Further, according to [2] Wolfe et al. “Dynamic control of optical-core/optical-cladding liquid waveguides”, PNAS, vol. 101, pp. 12434-12438, 2004, waveguides are known which e.g. use a liquid as a core which is surrounded by a solid material, and waveguides which comprise a liquid core and liquid cladding.
EW cells are designed and controlled in various ways to suit individual display applications. For example, [3] Blankenbach et al. “Novel highly reflective and bistable electrowetting displays”, Journal of the SID, vol. 16, pp. 237-244, 2008, shows a highly reflective display device which is binary controllable. When a voltage is applied to a structure of control electrodes, a tinted droplet is moved from one position to another position in a liquid (e.g. oil) within an EW cell. This allows a display of low information density to be realised, e.g. a bar graph display.
Further, a pixelated amplitude display whose EW pixels have an absorbing oil film and a transparent water film is known from [4] Feenstra et al. “Liquavista electrowetting displays”, white paper, www.liquavista.com. The oil film is moved aside within an EW pixel when applying a voltage, whereby the EW pixel becomes transparent at the positions where the oil film has been removed, and where the transmitted light can be modulated.
The individual physical forms of the quoted prior art will be detailed again in conjunction with the description of embodiments of the present invention.